Ice based NAT traversal

ABSTRACT

An originating P-CSCF node receives a SIP INVITE request from first user equipment (UE) that originates a call to a second UE. If a relay candidate address for the first UE is not present in the SIP INVITE request, the SIP INVITE request is modified to include a first address provided by an originating IMS-AGW node as a relay candidate for the first UE and forwarded to the second UE. The originating P-CSDF node receives a SIP INVITE response message from the second UE in response to the SIP INVITE request. If a relay candidate address for the second UE is not present in the SIP INVITE response, the SIP invite response is modified to include a second address provided by an originating IMS-AGW node as a relay candidate for the second UE and forwarded t the first UE. The address information is used by both UEs in ICE operations.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. §371 national stage application of PCTInternational Application No. PCT/EP2012/050663, filed on 17 Jan. 2012,the disclosure and content of which is incorporated by reference hereinhi its entirety.

TECHNICAL FIELD

The present invention relates to methods and apparatus in acommunication network for session establishment between user equipmentbehind network address translation (NAT) devices. More particularly, theinvention relates to methods and apparatus for establishing a multimediasession in a communication network including an Internet Protocol (IP)Multimedia Subsystem (IMS) when user equipment are located behind NATdevices.

BACKGROUND

Internet Protocol Multimedia (IPMM) services provide a dynamiccombination of voice, video, messaging, data, etc, within the same callor media session (multimedia session). By growing the numbers of basicapplications and the media which it is possible to combine, the numberof services offered to the end users will grow, and the inter-personalcommunication experience will be enriched. This will lead to a newgeneration of personalised, rich multimedia communication services,including so-called “combinational IP Multimedia” services.

The IP Multimedia Subsystem (IMS) network (also referred to as IMS) isthe technology defined by the Third Generation Partnership Project(3GPP) to provide IPMM services over fixed and mobile communicationnetworks. IMS provides key features to enrich the end-userperson-to-person communication experience through the integration andinteraction of services. IMS allows new rich person-to-person(client-to-client) as well as person-to-content (client-to-server)communications over Internet Protocol (IP)-based networks. The IMS makesuse of the Session Initiation Protocol (SIP) to set up and control callsor multimedia sessions between user equipment and application servers.The Session Description Protocol (SDP), carried by SIP signalling, isused to describe and negotiate the media components of the call ormultimedia session. In addition to SIP, other protocols may be used formedia transmission and control, such as Real-time Transport Protocol andReal-time Transport Control Protocol (RTP/RTCP).

A user equipment (UE) may comprise or represent any device used forcommunications over an IP-based communications network. Examples of UEsthat may be used in certain embodiments of the described communicationor access networks are fixed, wired or wireline devices, or mobile orwireless devices for accessing an IP based communication network, thedevices may include, but are not limited to, computers, terminals,telephones, mobile handsets, mobile phones, smart phones, portablecomputing devices such as lap tops, handheld devices, tablets,net-books, computers, personal digital assistants, customer premisesequipment, modems and other communication devices that may access an IPbased communication network.

FIG. 1 a illustrates schematically a communication network 100 in whicha first UE 101 originates a call or multimedia session towards a secondUE 103, the first UE 101 is located in an originating network 102 andthe second UE 103 is located a terminating network 104. The first UE 101and second UE 103 may communicate with each other through IMS network105. The first UE 101 accesses the IMS network 105 via a first accessnetwork 106 within the originating network 102, and the second UE 103accesses IMS network 105 via a second access network 107 within theterminating network 104. In addition, in the communication path betweenthe first UE 101 and the IMS network 105 there is a first NAT device 108in the originating network 102. Similarly, in the communication pathbetween the second UE 103 and the IMS network 105 is a second NAT device109 in the terminating network 104.

First and second NAT devices 108 and 109 allow a single public IPaddress to be shared between a number of UEs or IP hosts. UEs behind NATdevices (e.g. the first UE 101 is behind the first NAT device 108 andthe second UE 103 is behind second NAT device 109) may be given IPaddresses that are in a private IP address space allocated by a systemadministrator, an operator, or operators of first and second accessnetworks 106 and 107. These private addresses may not be routable over apublic IP-based network, for example, the Internet or another operator'saccess network, for example, over IMS network 105 and othercommunications networks. The first and second NAT devices 108 and 109create temporary bindings between the public and private addressspace(s) on a per-connection basis. A binding is a mapping between thepublic address and port to a private address and port associated with aspecific transport, for example, the user datagram protocol (UDP) ortransmission control protocol (TCP).

The first and second access networks 106 and 107 may include any corenetwork or access network technology including, but not limited to,various support entities or nodes (not shown) such as various interfacenodes, access points, routers, LAN bridges, switches, base stations,switching centers, network gateways that provide an interface betweenthe first and second access networks 106 and 107 and the IMS network105. This will allow the first and second UEs 101 and 103 to communicatewith each other through IMS network 105.

When a calling party such as user A of the first UE 101 originates acall or initiates a multimedia session to or with a called party such asuser B of the second UE 103 the set-up process involves an originatingcall associated with the first UE 101 set up in the originating network102 and a terminating call associated with the second UE 103 set up inthe terminating network 104.

The terms “originating call” and “terminating call” may comprise orrepresent the session or connection set-up signalling in relation to thefirst UE 101 and the second UE 103, respectively. Examples oforiginating or terminating calls that may be used in certain embodimentsof the described network, include but are not limited to, the connectionset-up signalling enabling a communication connection to be made betweenuser A of the first UE 101 and user B of the second UE 103 in the twocall halves model. The originating call is the connection set-upsignalling for user A of the first UE 101 in relation to the originatingnetwork 102 in the first call half and the terminating call is theconnection set-up signalling for connecting the call with user B ofsecond UE 103 in relation to terminating network 104 in the second callhalf.

The IMS network 105 includes an originating IMS 110 associated with theoriginating network 102 and a terminating IMS 111 associated with theterminating network 104. The originating and terminating IMS networks110 and 111 may include network entities, nodes, or IMS network nodesthat send/receive signals to/from the first and second access networks106 and 107, respectively. These IMS network nodes connect with thefirst and second access networks 106 and 107 via the access networkgateway or switching center nodes. The IMS network nodes may includeCall/Session Control Function (CSCF) nodes, which operate as SIP proxieswithin the IMS network 105. The 3GPP architecture defines several typesof CSCF nodes: the Proxy CSCF (P-CSCF) node which is typically the firstpoint of contact within the IMS network 105 for UEs, which are SIPenabled; the Serving CSCF (S-CSCF) node provides services to the userthat the user is subscribed to; and the Interrogating CSCF (I-CSCF) nodewhose role is to identify the correct S-CSCF and to forward to thatS-CSCF a request received from a UE via P-CSCF node.

In this example, it is assumed that the first UE 101 is subscribed toIMS services, which include IMS voice services, messaging, video,multimedia etc. When the first UE 101 originates a call or multimediasession with the second UE 103, the first UE 101 will be the callingparty and the call signalling of the first call half is the originatingcall in relation to the first UE 101. As the first UE 101 will use IPaddressing, since this will be an IP-based call or multimedia session,the call set-up signalling will be directed from the first UE 101 viathe first NAT 108 to the originating IMS network 110 in the originatingnetwork 102. As the second UE 103 is located in the terminating network104, IMS network 110 sends the call set-up signalling to IMS network 111for setting up the call signalling towards the called party, which isthe second UE 103, and the call signalling of the second call half, i.e.the terminating call in relation to the second UE 103 is directed viathe second NAT device 109 to second UE 103. The first and second UEs 101and 103 will communicate through the IMS network 105 using SIP messagingto set up and control calls or multimedia sessions.

However, for SIP and SDP messages sent through the IMS network 105, theIP address located in the SIP Contact header and the SDP connectionaddress (c-line) is usually identical because UEs will send SIP messagesfrom the same IP address that they want to receive media at. Within theIMS network 105, SIP signalling and the multimedia session do nottraverse the same network nodes as they are transported end-to-endindependently of each other. The first and second NAT devices 108 and109 will usually not be aware of the complex relations between thedifferent signalling protocols and may not take these relations intoaccount when performing IP address translation. This means SIPsignalling, which the IMS network 105 uses for most call set-upsignalling, may not be able to be used for UEs behind first and secondNAT devices 108 or 109.

NAT traversal mechanisms allow a UE to find out whether it is behind aNAT device and to become aware of the public transport addresses (IPaddresses and ports), such as public IP addresses and the publicaddresses of the remote end, which is the second UE in terminatingnetwork 104. The 3GPP Technical specification TS 24.229 specifies twoNAT traversal mechanisms that will allow SIP signalling end-to-end forsetting up multimedia sessions between UEs behind a NAT device usingSIP. These are referred to as Interactive Connection Establishment (ICE)(or UE Managed NAT Traversal) and the Hosted NAT traversal (or NetworkManaged NAT Traversal).

FIGS. 1 b and 1 c are a schematic diagram and a signal flow diagram,respectively, illustrating an example of an ICE NAT traversal betweenthe first UE 101 and the second UE 103 via originating and terminatingnetworks 102 and 104. The first UE 101 and second UE 103 include a SIPport and a media port, the SIP port is used for SIP signalingrepresented by solid arrows, and the media port is the interface to themedia bearers represented by dashed arrows for transporting traffic ormedia. It is assumed that the first and second UE 101 and 103 supportsthe ICE based NAT traversal.

Prior to initiating a multimedia session and setting up a multimediastream, the first UE 101 interacts with a Traversal Using Relays aroundNAT (TURN) server 112 a in the communication network 100 to discover apublic transport address (IP address and port number) that the TURNserver 112 a may allocate to the first UE 101 such as IP address A3. TheTURN server 112 a provides relay functionality within communicationsnetwork 100 so media can traverse first NAT device 108 via the TURNserver 112 a. The first UE 101 interacts with a Session TraversalUtilities NAT (STUN) server 113 a to discover the public address, e.g.IP address A2, when first UE 101 is located behind first NAT device 108.

Given the transport address information from the TURN server 112 aand/or STUN server 113 a, the first UE 101 originates the call bysending towards the second UE 103, a SIP INVITE (call setup request)request message with an SDP offer including candidate addressinformation based on the transport address information. The SIP INVITErequest message traverses communication network 100 via an originatingIMS-P-CSCF node 114 a of the originating network 104 and terminatingIMS-P-CSCF node 114 b of terminating network 104.

In this example, the candidate address information includes threetransport address candidates for the first UE 101, which are a relaytransport address candidate for the first UE 101 (e.g. IP address A3from TURN server 112 a), a server reflexive address candidate for thefirst UE 101 (e.g. IP address A2 from STUN server 113 a), and a localhost address transport address candidate for the first UE 101 (e.g. IPaddress A). If only one of the endpoints (UEs) supports ICE, the TURNrelay transport address will always be used (assuming there is a TURNserver available in the network).

On receiving the SIP INVITE request message and the SDP offer from thefirst UE 101, the second UE 103 interacts with TURN and STUN servers 112b and 113 b to gather candidate address information for the second UE103 in the same manner as the first UE 101. The second UE 103 transmitstowards the first UE 101, a SIP INVITE (call setup response) responsemessage including an SDP answer (session description) including thecandidate address information via terminating IMS-P-CSCF node 114 b andthe originating IMS-P-CSCF node 114 a. The candidate address informationfor the second UE 103 includes three transport address candidates, whichare a relay transport address candidate for the second UE 103 (e.g. IPaddress B3 from TURN server 112 b), the server reflexive addresscandidate for the second UE 103 (e.g. IP address B2 from STUN server 113b), and the local host address candidate for the second UE 103 (e.g. IPaddress B).

After receiving the corresponding candidate address information fromeach other, the first and second UEs 101 and 103 perform the ICEprocedure. In this procedure, the first and second UEs 101 and 103 sendICE connectivity checks to the transport address candidates receivedfrom the other UE 103 or 101 (endpoint). If only one endpoint is behindNAT device 108 or 109, or the involved NAT device(s) 108 and/or 109 arenot address restrictive, then either the connectivity checks on the hostaddress candidate(s) or the server reflexive address candidate(s) willsucceed. This means TURN servers 112 a and 112 b are not required. Inthis case, the UEs 101 and 103 interact with TURN servers 112 a and 112b to de-allocate the relay resources the TURN servers 112 a and 112 btentatively previously reserved.

For ICE based NAT traversal, to not always result in using a TURN server112 a or 112 b for relaying the traffic requires the first and secondUEs 101 and 103 to support the ICE mechanism, or the IMS network 105 hasto act as a back-to-back user agent (B2BUA) with regards to the ICEmechanism. However, there may also be many UEs that do not support ICEfunctionality such as legacy UEs and even some newer UEs. For NATtraversal to be possible for the UEs that do not support ICE and whereNAT traversal is not solved on customer premises (through use ofintelligent NAT devices such as Universal Plug and Play (UpNP) or SIPapplication layer gateway (ALG) based NAT traversal), IMS supportsanother NAT traversal mechanism called Hosted NAT traversal, which isrelay based and similar to TURN server based NAT traversal.

FIGS. 1 d and 1 e are a schematic diagram and a signal flow diagram,respectively, illustrating an example of a Hosted NAT traversalmechanism between the first UE 101 and the second UE 103 via originatingand terminating networks 102 and 104. The first and second UEs 101 and103 include a SIP port and a media port, the SIP port is used for SIPsignaling represented by solid arrows, and the media port is theinterface to the media bearers etc represented by dashed arrows. It isassumed that the first and second UE 101 and 103 do not support the ICEbased NAT traversal mechanism.

In the Hosted NAT traversal mechanism, the originating and terminatingIMS P-CSCF nodes 114 a and 114 b perform NAT traversal by manipulatingthe transport address information in the media descriptions of the SDPoffer and answer exchanged in the SIP signalling between the first andsecond UE 101 and 103. In doing so, the IMS P-CSCF nodes 114 a and 114 bplace or insert IMS Access Gateway (IMS AGW) nodes 115 a and 115 b intothe communication path between the first and second UEs 101 and 103 sothat the media session is relayed via the IMS-AGW nodes 115 a and 115 b.

Each of the IMS-AGW nodes 115 a and 115 b, will, if there is a NATdevice 108 or 109 between it and the corresponding UE 101 or 103, doHosted NAT traversal. This means the IMS-AGW node 115 a and 115 bdiscover the transport address (and port) used on the IMS side of thefirst and second NAT devices. This is performed by inspecting the sourcetransport address information in the first packet received from each UE101 and 103, respectively. This source transport address information isused as the destination transport address information for packetsrelayed in the other direction.

In this way, a multimedia session is set up between the first UE 101 andthe second UE 103 such that the first and second NAT devices 108 and 109are not required to manipulate the SIP signalling. However, this meansthat the communication path between the first and the second UEs 101 and103 includes first and second NAT devices 108 and 109 and originatingand terminating IMS-AGW nodes 115 a and 115 b. All of these devices andnodes need to perform address translation to allow multimedia sessionpackets to be transmitted/received by the first and second UEs 101 and103 resulting in increasing delays for multimedia packets traversing thecommunication path between the first and second UEs 101 and 103.

There are many reasons why the IMS P-CSCF nodes 114 a and 114 b mayroute multimedia sessions via an IMS-AGW nodes 115 a and 115 b. It maybe due to a general security policy (e.g. authentication purposes), orrequired for mapping a specific multimedia session between IPv6transport and IPv4 transport, or required for NAT traversal and at leastone of the first and/or second UEs 101 and/or 103 does not support ICE,or for any other reason etc. However, if the first UE 101 and/or thesecond UE 103 uses ICE based NAT traversal and the IMS P-CSCF nodes 114a and 114 b route the multimedia session via an IMS-AGW nodes 115 a and115 b, then the ICE mechanism would result in choosing the relaycandidate address (from TURN server 112 a and/or 112 b). This results ina TURN server 112 a and/or 112 b in series with IMS-AGW nodes 115 aand/or 115 b. This means the communications path between the first andsecond UEs 101 and 103 may include first and second NAT devices 108 and109, TURN servers 112 a and 112 b, and IMS AGW nodes 115 a and 115 b,resulting in increasing delays for multimedia packets traversing thecommunication path between the first and second UEs 101 and 103.

This can be alleviated if the IMS P-CSCF nodes 114 a and 114 b canterminate the ICE signalling in the SIP signalling and SDP offer/answerbodies while the IMS-AGW nodes 115 a and 115 b terminates the ICEconnectivity checks in the media plane. This is illustrated in FIG. 1 f,which is a signalling flow diagram illustrating an example of a combinedNAT traversal mechanism incorporating Hosted NAT traversal and ICE basedNAT traversal. As specified in 3GPP TS 24.229, the IMS AGW nodes 115 aand 115 b can terminate the ICE signalling by replacing the ICEmechanisms address candidates in the SDP offer/answer bodies with hostcandidate addresses provided by the corresponding IMS AGW nodes 115 aand 115 b (e.g. IP addresses T1, T2, T3, or T4).

This means the communication network 100 will use Hosted NAT traversaland even if ICE mechanism is supported and used by the UEs 101 and 103,then the host candidate address will always be selected by the ICEmechanism, so there would never be both an IMS-AGW node and a TURNserver in the established end-to-end media connection. However, thisstill means that the communication path between the first and the secondUEs 101 and 103 will still include first and second NAT devices 108 and109 and IMS-AGW nodes 115 a and 115 b, which need to perform addresstranslation to allow multimedia session packets to betransmitted/received by the first and second UEs 101 and 103. This stillresults in delays for multimedia packets traversing the communicationpath between the first and second UEs 101 and 103 through first andsecond NAT devices 108 and 109 and IMS-AGW nodes 115 a and 115 b.

An operator of an IMS network 105, or of originating IMS network 110, orof terminating IMS network 111 may need to deploy and manage TURNservers 112 a and/or 112 b and IMS-AGWs/Translation Gateway (TrGW) nodes115 a and/or 115 b if their policy is not to always anchor media fromfirst and/or second UEs 101 and/or 103 behind first and/or second NATdevices 108 and/or 109 via IMS-AGW nodes 115 a and/or 115 b,respectively. However, it is inevitable that current NAT traversalmechanisms can result increased or unnecessary delays for multimediapackets traversing the communication path between the first and secondUEs 101 and 103. With the increasing use of high bandwidth multimediaapplications, these delays will be unacceptable for time sensitivereal-time multimedia traffic such as multimedia streaming, voice, andvideo conferencing applications.

There is a desire for a NAT traversal mechanism for use within an IMSnetwork that minimises the number of nodes required for NAT traversalwithin the media communication path of a multimedia session.

SUMMARY

It is an object of the present invention to provide a mechanism forperforming ICE based NAT traversal within a communications network inorder to minimise or prevent an unnecessary use of nodes or relayservers in the communication path of a media session between endpoints,while also not requiring TURN servers to be deployed.

A first aspect of the present invention provides a method for operatinga call originating P-CSCF node for an ICE based NAT traversal in acommunications network including an IMS, a first UE and a second UE. ASIP Invite message originating from the first UE is received at theoriginating P-CSCF node. The SIP Invite message includes candidateaddress information for the first UE. If a server reflexive candidateaddress for the first UE is present in the candidate address informationfor the first UE, and, if a relay candidate address for the first UE isnot present in the candidate address information for the first UE, thenthe SIP Invite message is modified by including in the candidate addressinformation for the first UE a first address provided by an originatingIMS Access Gateway node as the relay candidate address for the first UE.The modified SIP Invite message is forwarded to a further IMS node forrouting the SIP Invite message towards the second UE. Otherwise, thereceived SIP Invite message is forwarded to the further IMS node forrouting the SIP Invite message towards the second UE. The candidateaddress information for the first UE is for use by the second UE whenperforming ICE procedures.

As an option, a SIP Invite response message originating from the secondUE is received at the originating P-CSCF node, the SIP Invite responsemessage includes candidate address information for the second UE. If aserver reflexive candidate address for the second UE is present in thecandidate address information for the second UE, and, if a relaycandidate address is not present in the candidate address informationfor the second UE, then the SIP Invite response message is modified byincluding in the candidate address information for the second UE asecond address provided by the originating IMS AGW node as the relaycandidate address for the second UE. The modified SIP Invite responsemessage is forwarded to the first UE. Otherwise, the received SIP Inviteresponse message is forwarded to the first UE. The candidate addressinformation for the second UE is for use by the first UE when performingICE procedures.

Optionally, the second address of the originating IMS AGW node isdetermined for use as the relay candidate address. Alternatively oradditionally, when the first UE is behind a first NAT device, then theoriginating IMS AGW node is instructed by the originating P-CSCF node toperform address latching towards the first UE for discovering thetransport address the first NAT device uses for the first UE.Alternatively or additionally, when the second UE is behind a second NATdevice, the originating IMS AGW node is instructed by the originatingP-CSCF node to perform address latching towards the second UE fordiscovering the transport address the second NAT device uses for thesecond UE.

Optionally, modifying the received SIP Invite message may furtherinclude setting the relay candidate address for the first UE, which isthe first address of the originating IMS AGW node, to be the defaultcandidate address. Additionally, an update message can be received fromthe first or second UE, being the ICE controlling endpoint, the updatemessage indicating that another address candidate other than the relaycandidate address for the first UE has been selected for use. The otherselected address candidate is set as the default candidate address andthe first address of the originating IMS-AGW node is removed, such thatthe originating IMS-AGW node is removed from the media path between thefirst and second UE.

According to a second aspect of the invention there is provided a methodfor operating a terminating call P-CSCF node for ICE based NAT traversalin a communications network including an IMS, a first UE, and a secondUE. A SIP Invite message originating from the first UE for the second UEis received at the terminating P-CSCF node, the SIP Invite messageincludes candidate address information for the first UE. The receivedSIP Invite message is forwarded towards the second UE. The candidateaddress information for the first UE is for use by the second UE whenperforming ICE procedures.

Optionally, in response to the SIP Invite message originating from thefirst UE, a SIP Invite response message originating from the second UEis received at the terminating P-CSCF node. The SIP Invite responsemessage includes candidate address information for the second UE. If aserver reflexive candidate address is present in the candidate addressinformation for the second UE, and if a relay candidate address for thesecond UE is not present in the candidate address information for thesecond UE, and if a relay candidate address for the first UE was notpresent in the associated received SIP Invite message candidate addressinformation for the first UE, then the SIP Invite response message ismodified by including in the candidate address information for thesecond UE a second address of the terminating IMS Access Gateway node asthe relay candidate address for the second UE. The modified SIP Inviteresponse message is forwarded to another IMS node for routing themodified SIP Invite response message towards the first UE. Otherwise,the received SIP Invite response message is forwarded to another IMSnode for routing the received SIP Invite response message towards thefirst UE. The candidate address information for the second UE is for useby the first UE when performing ICE procedures.

As an option, the second address of the terminating IMS AGW node for useas the relay candidate address for the second UE is determined.Alternatively or additionally, when the first UE is behind a first NATdevice, then the terminating IMS AGW node is instructed by theterminating P-CSCF node to perform address latching towards the first UEfor discovering the transport address the first NAT device uses for thefirst UE. Alternatively or additionally, when the second UE is behind asecond NAT device, terminating IMS AGW node is instructed by theterminating P-CSCF node to perform address latching towards the secondUE for discovering the transport address the second NAT device uses forthe second UE.

As an option, modifying the received SIP Invite response message mayfurther include setting the relay candidate address for the second UE,which includes the second address provided by the terminating IMS AGW,to be the default candidate address. Additionally, an update message maybe received from the first or second UE, being the ICE controllingendpoint, the update message indicating that another address candidateother than the relay candidate address for the first or second UE hasbeen selected for use. The another address candidate is set as thedefault candidate address. The relay candidate address for the first orsecond UE is removed such that the originating or terminating IMS-AGWnode is removed from the media path between the first and second UE.

According to a third aspect of the invention there is provided a networknode in a communications network including an IMS, a first UE, and asecond UE. The network node includes a receiver, a transmitter, a memoryunit, and a processor, the processor being connected to the receiver, tothe transmitter, and to the memory unit. The receiver is configured forreceiving a SIP Invite message from the first UE for originating a mediastream with the second UE. The received SIP Invite message comprising atleast a host candidate address and a server reflexive candidate addressfor the first UE. The processor is configured for modifying the receivedSIP Invite message to include the address of an originating IMS AGW nodeas a relay candidate address for the first UE when a relay candidateaddress for the first UE is not present in the received SIP Invitemessage. The transmitter is configured for transmitting the modified SIPInvite message to a terminating call P-CSCF node associated with thesecond UE, the candidate address for the first UE for use by the secondUE when performing ICE procedures.

As an option, the receiver is further configured for receiving a SIPInvite response message from the terminating call P-CSCF node, the SIPInvite response message including at least a host candidate address anda server reflexive candidate address for the second UE. The processor isfurther configured for modifying the received SIP Invite responsemessage to include the originating IMS AGW node as the relay candidateaddress for the second UE when the relay candidate address for thesecond UE is not present in the received SIP Invite response message.The transmitter is further configured for transmitting the modified SIPInvite response message to the first UE for use by the first UE whenperforming the ICE procedure with the second UE.

According to a fourth aspect of the invention there is provided anetwork node in a communications network including an IMS, a first userUE, a second UE, the network node including a receiver, a transmitter, amemory unit, and a processor, the processor being connected to thereceiver, to the transmitter, and to the memory unit. The receiver isconfigured to receive a SIP Invite message originating from the first UEfor the second UE. The SIP Invite message includes candidate addressinformation for the first UE. The transmitter is configured to forwardthe received SIP Invite message towards the second UE. The candidateaddress information for the first UE is for use by the second UE whenperforming ICE procedures.

As an option, the receiver is further configured to receive a SIP Inviteresponse message originating from the second UE in response to the SIPInvite message originating from the first UE. The SIP Invite responsemessage includes candidate address information for the second UE. If aserver reflexive candidate address is present in the candidate addressinformation for the second UE, and if a relay candidate address for thesecond UE is not present in the candidate address information for thesecond UE, and if a relay candidate address for the first UE was notpresent in the associated received SIP Invite message candidate addressinformation for the first UE, then the processor is further configuredto modify the SIP Invite response message candidate address informationfor the second UE to include a second address provided by theterminating IMS Access Gateway node as the relay candidate address forthe second UE. The transmitter is further configured to forward themodified SIP Invite response message to another IMS node for routing themodified SIP Invite response message towards the first UE. Otherwise,the transmitter is further configured to forward the received SIP Inviteresponse message to another IMS node for routing the received SIP Inviteresponse message towards the first UE. The candidate address informationfor the second UE is for use by the first UE when performing ICEprocedures.

Embodiments of the present invention can provide a relatively simple andefficient mechanism for providing an efficient use of communicationresources, for example, minimising the deployment of relay servers suchas TURN servers, and providing an efficient throughput for traffic in amedia session when NAT traversal is required. This provides a way foroperators to provide NAT traversal functions that allow UEs to beconfigured to select the most optimal communication or media path fortraffic of a media session.

As an example, in a communications network when NAT traversal isrequired, and when a UE supports ICE, and there is no other reason foran IMS-AGW node to be in the communication path than for NAT traversal,then in the absence of relay candidates in the SDP information, thepresent invention allows the IMS network to add only an IMS-AGW node asrelay candidate by manipulating SDP information in the call set-up SIPsignalling. This provides the advantage that only an IMS-AGW node willbe in the eventually established call path if the ICE mechanism resultsin the relay candidate being used (e.g. the host and server reflexivecandidate addresses did not work).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a illustrates schematically a communications network including anoriginating network and a terminating network;

FIG. 1 b illustrates schematically a portion of an originating networkfor use in an example ICE NAT traversal mechanism;

FIG. 1 c illustrates a signalling flow diagram for an example ICE NATtraversal mechanism;

FIG. 1 d illustrates schematically a portion of an originating networkfor use in an example Hosted NAT traversal mechanism;

FIG. 1 e illustrates a signalling flow diagram for an example Hosted NATtraversal mechanism;

FIG. 1 f illustrates a signalling flow diagram for an example Hosted NATtraversal with a UE using the ICE mechanism;

FIG. 2 a illustrates schematically a portion of an originating networkfor use in an example solution for operating an originating orterminating IMS P-CSCF node for ICE based NAT traversal according to theinvention;

FIG. 2 b illustrates a signalling flow diagram for another examplesolution for operating an originating or terminating IMS P-CSCF node forICE based NAT traversal according to the invention;

FIG. 3 a illustrates a flow diagram for an example solution foroperating an originating IMS P-CSCF node for ICE based NAT traversalaccording to the invention;

FIG. 3 b illustrates a flow diagram for another example solution foroperating an originating IMS P-CSCF node for ICE based NAT traversalaccording to the invention;

FIG. 3 c illustrates a flow diagram for an example solution foroperating a terminating IMS P-CSCF node for ICE based NAT traversalaccording to the invention;

FIG. 3 d illustrates a flow diagram for another example solution foroperating a terminating IMS P-CSCF node for ICE based NAT traversalaccording to the invention;

FIG. 4 illustrates a schematic diagram for an example of a network nodeaccording to the invention;

FIG. 5 illustrates a schematic diagram for another example of a networknode according to the invention.

DETAILED DESCRIPTION

FIGS. 1 b and 1 c illustrate a first scenario for NAT traversal when theUE supports ICE and the IMS core network does not insert an IMS-AGW nodein the media path. The ICE based NAT traversal mechanism results inrelay based NAT traversal using TURN servers for those NAT traversalcases where a media relay node is either the only means to achieve NATtraversal or when the other endpoint does not support the ICE mechanism.FIGS. 1 d, 1 e, and 1 f illustrate a second scenario for NAT traversalwhen the IMS network detects a NAT device located between the UE and theIMS core network. When either A) the UE does not support ICE based NATtraversal (e.g. as illustrated in FIGS. 1 d and 1 e), or B) the UEsupports ICE based NAT traversal, but the IMS-AGW node is already usedin the media path (e.g. as illustrated in FIG. 1 f), then multipleIMS-AGW nodes are used as media relay nodes in the IMS network, which iscommonly used to avoid TURN based NAT traversal.

In order to at least partially overcome the problems described above andherein it is proposed to improve the ICE based NAT traversal mechanismby efficiently using IMS-AGW nodes in place of TURN servers whereverpossible. This would allow an IMS operator not to deploy severaldifferent types of relay servers or nodes (e.g. TURN servers forsupporting the first scenario and IMS-AGW nodes for supporting thesecond scenario), but to deploy only IMS-AGW nodes. The ICE basedmechanism as proposed herein can minimise the number of relay servers,media relay or IMS AGW nodes placed in the media path or communicationpath further reducing latencies for traffic in a media session between afirst UE and a second UE.

As an example, the mechanism can be a method and/or apparatus foroperating a call originating P-CSCF node for ICE based NAT traversalwhen a first UE originates a call with a second UE. The originating callP-CSCF receives a call session request from the first UE, the callsession request including candidate address information (e.g. a hostcandidate address, or a server reflexive candidate address, or a relaycandidate address) for the first UE. The candidate addresses may betransport addresses that comprise IP address(es), or IP address(es) andport(s). If a relay candidate address for the first UE is not present inthe call session request, the originating P-CSCF node modifies the callsession request to include a first address provided by an originatingIMS AGW node as the relay candidate address for the first UE (e.g. ana-line i.e. a=candidate first address provided by IMS AGW addressrelay). This means an originating IMS AGW node is included to perform asa media relay server in a list of transport address candidates for useby the second UE when performing ICE procedures (e.g. ICE connectivitychecks). The modified call session request is transmitted to a furtherIMS node for routing towards the second UE. Otherwise, when a relaycandidate is present or when only a host candidate address is present,then the received call session request is forwarded to the further IMSnode for routing towards the second UE. The transport address candidatesfor the first UE in the call session request are used by the second UEwhen performing ICE procedures.

In addition, a mechanism such as a method and/or apparatus for operatinga call terminating P-CSCF node for ICE based NAT traversal when thefirst UE originates a call with the second UE. The call terminatingP-CSCF node receives the call session request from the first UE, thecall session request including candidate address information (e.g. ahost candidate address, or a server reflexive candidate address, or arelay candidate address) for the first UE. The terminating P-CSCF nodeforwards the call session request from the first UE to the second UE.The candidate address information for the first UE is used by the secondUE in ICE procedures. In response, the second UE gathers candidateaddress information (e.g. a host candidate address, or a serverreflexive candidate address, or a relay candidate address) for thesecond UE. The terminating P-CSCF node receives a call session responsefrom second UE in response to the second UE receiving the call sessionrequest from the first UE. The terminating P-CSCF node will modify thecall session response if a relay candidate address for the second UE isnot present in the call session response from the second UE and if arelay candidate address for the first UE was not present in thecorresponding call session request from the first UE. If theseconditions are satisfied, then the terminating P-CSCF node modifies thecall session response to include a second address provided by aterminating IMS AGW node as the relay candidate address for the secondUE (e.g. an a-line i.e. a=candidate second address provided by IMS AGWaddress relay). This means a terminating IMS AGW node is included toperform as a media relay server in a list of transport addresscandidates for use by the first UE when performing ICE procedures (e.g.ICE connectivity checks). The modified call session response istransmitted to a further IMS node for routing towards the first UE.Otherwise, the received call session request is forwarded to the furtherIMS node for routing towards the first UE. The transport addresscandidates for the second UE in the call session response are used bythe first UE when performing ICE procedures.

In order for the first and second UE to complete their configuration ofthe media and communication paths, the originating P-CSCF node shouldreceive a call session response from the second UE that is associatedwith the call session request from the first UE. The originating P-CSCFnode may receive a call session response originating from the second UE,where the call session response includes candidate address informationfor the second UE (e.g. a host candidate address and/or a serverreflexive candidate address and/or a relay candidate address for thesecond UE). If the relay candidate address for the second UE is notpresent in the received call session response, but there are transportcandidate attributes in the response, then the originating call P-CSCFnode modifies the candidate address information for the second UE in thereceived call session response to include a second address provided bythe originating IMS AGW node as the relay candidate address for thesecond UE. The modified call session response is forwarded to the firstUE. Otherwise, the received call session response is forwarded to thefirst UE as is. The first UE uses the candidate address information forthe second UE when performing the ICE procedure.

This means that if a relay server is needed for NAT traversal in bothdirections then the media path between the first and second UE includesonly the originating IMS AGW as a relay server, when no other relayservers are present. If a relay server is not needed for NAT traversalin both directions, then the ICE procedure results in that there will beno relay server at all in the communication path eventually established.

When a relay candidate address for the first UE is present in thereceived call session request, the originating call P-CSCF node forwardsthe received call session request towards a further IMS node for routingto the second UE. This means that a TURN server has been discovered bythe first UE and so the originating P-CSCF should not place a furtherrelay server, e.g. should not place an IMS AGW, into the communicationor media path between the first and second UEs. During ICE connectivitychecks, if a relay server is required by the first UE then only the TURNserver will be included in the communications path, not an IMS AGW aswell. When a relay candidate address for the second UE is present in thereceived call session response, the originating call P-CSCF nodeforwards the received call session response towards the first UE. Thismeans that a TURN server has been discovered by the second UE and so theoriginating and/or terminating P-CSCF should not place a further relayserver, e.g. an IMS AGW, in the communication or media path between thesecond and the first UEs. During ICE connectivity checks, if a relayserver is required by the second UE then only the TURN server will beincluded in the communications path between the second and first UE, notan IMS AGW as well.

In addition, when the call session request and call session response aremodified by the originating IMS AGW node to include a relay candidate,the relay candidate address (e.g. the address of the originating IMSAGW) is set to be the default candidate address. When the ICEconnectivity checks have completed between the first and second UEs, theICE controlling endpoint (either the first or second UE) may transmit anupdate message to the originating P-CSCF node and/or the terminatingP-CSCF node, the update message indicating the candidate pair ofaddresses that were selected by the first and second UEs for configuringthe media or communication path between the first and second UEs. If thedefault candidate address is not included in the candidate pair ofaddresses, then the originating P-CSCF may remove or release theoriginating IMS-AGW from the media path between the first and second UE.

The above-described example solution according to the invention usedcall session request and response messages. Although the followingexample solutions according to the invention are described with respectto the SIP signalling protocol and SDP offer/answer protocols andmodels, it is to be appreciated that other signalling protocols or callsession protocols and session description protocols may be used in placeof the SIP signalling and SDP protocols when a first UE originates acall with a second UE.

FIGS. 2 a and 2 b are schematic and signalling flow diagramsillustrating an example solution according to the invention foroperating originating and terminating P-CSCF nodes 114 a and 114 b forICE based NAT traversal in communications network 100. The referencenumerals for FIGS. 1 a to 1 e will be reused for the same or similarnetwork elements. This example solution eliminates the need for, butdoes not exclude the use of, TURN servers when using ICE based NATtraversal in communications network 100 in an IMS network environment.

When the first UE 101 originates a call towards a second UE 103, anoperator may desire the media connection between the first and secondUEs 101 and 103 not to go through a relay server unless this is the onlyway to achieve NAT traversal. If there are no TURN servers available tobe contacted by an ICE compliant UE, such as first UE 101 and/or secondUE 103 that is behind a NAT device 108 and/or 109, respectively, the IMSnetwork 105 will be configured, e.g. using originating P-CSCF node 114 aand/or terminating P-CSCF node 114 b, to use an IMS-AGW node 115 aand/or 115 b as a substitute relay server where a TURN server may beused.

The ICE procedures at the first and second UEs 101 and 103 will allowthe first and second UEs 101 and 103 to select the optimal media pathsuch that in situations where NAT traversal can be achieved without amedia relay then no IMS-AGW node 115 a and/or 115 b will be used. An IMSnetwork operator would never need to deploy TURN servers. Instead, basedon the example solution as described below, the operator could deployIMS-AGW nodes 115 a/115 b and configure the originating and terminatingP-CSCF nodes 114 a/114 b to efficiently employ the use of IMS-AGW nodes115 a/115 b for NAT traversal sparingly to minimize the number of relayservers within the media path. Although the originating and terminatingP-CSCF nodes 114 a/115 b may use the H.248 Gateway networking protocolwhen communicating with the IMS AGW nodes 115 a/115 b, it is to beappreciated that any network protocol may be used to communicate withnodes having IMS AGW functionality.

Referring to FIGS. 2 a and 2 b, the first UE 101 is assumed to be an ICEcompliant UE 101 behind the first NAT device 108 of the originatingnetwork 102. The first and second UEs 101 and 103 include a SIP port anda media port, the SIP port is used for SIP signaling represented bysolid arrows, and the media port is the interface to the media bearersetc represented by dashed arrows. Prior to originating a call orinitiating a multimedia session and setting up a multimedia stream, instep 200, the first UE interacts with a first STUN server 113 a todetermine the transport address(es) that the first NAT device 108 oforiginating network 102 would allocate for the media on its publicnetwork side when the first NAT device 108 does not perform address orport restrictive filtering on packets towards the first UE 101. Atransport address may include data representative of an IP address ordata representative of an IP address and port. Should the first UE 101also find a TURN server (not shown), the first UE 101 will then interactwith the TURN server to set up a tunnel between the first UE 101 and theTURN server, which will allocate relay transport address(es) for themedia to use on the public network side of the TURN server. In thiscase, the first UE 101 does not find a TURN server and so only discoversat least one server reflexive candidate transport address received inanswer from the STUN server 113 a.

In step 201, the first UE 101 uses SIP signaling to originate a calltowards the second UE 101 by sending a SIP INVITE request message withan SDP offer to the originating P-CSCF node 114 a for forwarding towardssecond UE 103. The SIP INVITE request message with the SDP offerincludes candidate address information for the first UE. The candidateaddress information includes at least the host candidate address of thefirst UE (e.g. the IP address and/or port information A), a serverreflexive candidate address for the first UE (e.g. IP address and/orport information A2) received in the answer from the STUN server), andoptionally a relay candidate address for the first UE received from aTURN server (not shown), should there be one. In this example, a TURNserver is not found by the first UE 101, so there will not be a relaycandidate address included in the candidate address information.

As shown in FIG. 2 b, because first UE 101 is behind first NAT device108, the candidate address information may include the list of SDPattributes c=A1; a=candidate A host; and a=candidate A2 srvrflx, whereA1 is the transport address provided by NAT device 108 in relation tothe host transport address, A is the host transport address, and A2 isthe transport address provided by the STUN server 113 a.

Referring to FIGS. 1 a to 1 e, 3GPP TS 24.229 requires that anoriginating/terminating IMS P-CSCF node 114 a/114 b receiving a SIPINVITE request message from a UE, when the first UE 101 is behind firstNAT device 108 and when the SDP offer for a media component contains ICEcandidate attributes (e.g. the above SIP INVITE request message), thenthe P-CSCF nodes 114 a/114 b may place IMS-AGW nodes 115 a/115 b in themedia path. This is generally determined by locally configured policies.However, if there are no reasons other than Hosted NAT traversal for theoriginating/terminating P-CSCF node 114 a/114 b to place an IMS-AGW node115 a/115 b in the media path, then the originating/terminating P-CSCFnode 114 a/114 b may choose either to A) not to place an IMS-AGW node115 a/115 b in the media path and instead rely ICE with STUN and TURNservers 113 a/113 b and 112 a/112 b, respectively, as in FIG. 1 c, or B)the originating/terminating P-CSCF node 114 a/114 b may place IMS-AGWnodes 115 a/115 b in the media path and command them to perform hostedNAT traversal as in FIG. 1 e. This causes media to always be routedthrough an IMS-AGW nodes 115 a/115 b including any TURN and STUN servers112 a/112 b and 113 a/113 b that may be available.

Referring to FIGS. 2 a and 2 b, an advantage provided by the examplesolution for ICE based NAT traversal is that the number of IMS-AGW nodes115 a/115 b acting as relay servers is greatly minimized, even when noTURN servers are available to the first UE 101. The example solutionensures that the option to use relay servers is maintained, and in mostcases reduces the number of IMS-AGW nodes 115 a/115 b and relay serversthat are needed in the media path between the first and second UEs forICE based NAT traversal.

Referring to FIG. 2 b, on receiving the SIP INVITE request message fromthe first UE 101, the originating P-CSCF node 114 a will check thecandidate address information to determine whether a relay candidateaddress for the first UE 101 is present. If a server reflexive candidateaddress is present, then this means that the first UE 101 is behindfirst NAT device 108, and if a relay candidate address is not present inthe candidate address information for the first UE 101, then step 202 ais performed. In step 202 a, the originating P-CSCF node 114 a(depending on other locally configured policies) adds or places anoriginating IMS-AGW node 115 a in the media path through manipulatingthe transport address information in c= and m= and a=lines of the SDPoffer. This can be achieved by having a relay candidate addresscorresponding to a first address provided by the originating IMS-AGWnode 115 a being added to the existing candidates in the SDP offer.

The candidate address information in the SIP INVITE request message ismodified to include the first address provided by the originatingIMS-AGW node 115 a as a relay candidate address. For example, in FIG. 2b, the candidate address information is modified to include the SDPattribute, a=candidate T2 relay, in which T2 is a transport addressprovided by originating IMS AGW node 115 a as the relay candidateaddress. The relay candidate address may also be set as the defaultcandidate. Alternatively, the originating P-CSCF node 114 a may trustthat NAT traversal will be achieved without a relay server and thereforenot place an IMS-AGW node 115 a in the path, which may not beguaranteed.

Once the originating IMS AGW node 115 a has been added, the originatingP-CSCF node 114 a may also instruct or command the originating IMS-AGWnode 115 a to perform address latching towards the first UE 101 and/orthe second UE 103. This is because the originating P-CSCF node 114 a maynot know the address (IP address and/or port) to which the originatingIMS AGW node 115 a will relay media packets. So IMS AGW node 115 a willneed to wait for media packets to arrive and inspect the source addressand use that address to relay media that is to be relayed in the otherdirection. In step 202 b, the originating P-CSCF node 114 a forwards themodified SIP INVITE request message to further IMS nodes in IMS network105 for routing towards the second UE 103.

If it is determined that a relay candidate address is present in thecandidate information for the received SIP INVITE request message, i.e.a media component of the SDP offer contains a TURN server relaycandidate, then the originating P-CSCF node 114 a will not invoke hostedNAT traversal with the originating IMS-AGW node 115 a for the mediacomponent. That is the received SIP INVITE request message isunmodified. This means originating IMS AGW node 115 a will not be placedin the media path. In this case, the originating P-CSCF node 114 aforwards the received SIP INVITE request message (unmodified SIP INVITErequest message) from UE 101 to further IMS nodes in IMS network 105 forrouting towards the second UE 103.

When the terminating call side P-CSCF node 114 b, referred to as theterminating P-CSCF node 114 b, receives a SIP INVITE request messageoriginating from the first UE 101. This may be the modified SIP INVITEmessage from the first UE as described above or the unmodified SIPINVITE message from the first UE as described above. The terminatingP-CSCF node 114 b performs a check on the received SIP INVITE message todetermine whether a relay candidate address is present in the SDP offer.Due to the presence of the relay candidate address in either themodified or unmodified SIP request message, the terminating P-CSCF node114 b does not place an IMS AGW node 115 b in the path. Instead, in step203, the terminating P-CSCF node 114 b makes note that the SIP INVITErequest message from first UE 101 had a relay candidate address andforwards the received SIP INVITE request message to the second UE 103 inthe terminating network 104. Should the SIP INVITE message from thefirst UE not include a relay candidate address, the terminating P-CSCFnode 114 b forwards the received SIP INVITE message towards the secondUE 103.

On receiving the SIP INVITE request message, the second UE 103 can usethe candidate address information for the first UE 101 when performingICE procedures. In step 204, after receiving the SIP INVITE requestmessage originating from the first UE 101, the second UE 103 performscandidate transport address discovery in the terminating network 104with STUN server 113 b (and if present a TURN server) in a similarfashion as the first UE 101 performed in step 200 in the originatingnetwork 102.

In step 205, in response to the SIP INVITE request message from thefirst UE 101, the second UE 103 sends a SIP INVITE response message withan SDP answer to the terminating P-CSCF node 114 b. The SDP answerincludes candidate address information for the second UE 103. As seen inFIG. 2 b, the candidate address information includes at least the hostcandidate transport address of the second UE 103 (e.g. IP address and/orport information B), a server reflexive candidate transport address forthe second UE 103 (e.g. IP address and/or port information B2) receivedin the answer from the STUN server 113 b, and optionally a relaytransport address (e.g. optional IP address and/or port information B3)for the second UE received from a TURN server (not shown). In FIG. 2 b,an attribute in square brackets is considered optional.

In the case no TURN server is found then no relay transport address isincluded in the candidate address information for the second UE 103. Asshown in FIG. 2 b, because the second UE 103 is behind a second NATdevice 109 in the terminating network 104, the candidate addressinformation may include the list of SDP attributes c=B1 (e.g. the publicaddress NAT device 109 uses for the second UE 103); a=candidate B host(actual host address of second UE 103); and a=candidate B2 srvrflx(server reflexive address provided by STUN server 113 b), where B1 isthe transport address provided by NAT device 109, B is the hosttransport address for the second UE 103, and B2 is the transport addressprovided to second UE 103 by the STUN server 113 b.

When the terminating P-CSCF node 114 b receives the SIP INVITE responsemessage with the SDP answer, the terminating P-CSCF node 114 bdetermines whether a relay candidate address is present in the SDPanswer, which, in this example, is not present. If it is determined thata relay candidate address for the second UE 103 is not present in thecandidate address information for the second UE 103, then theterminating P-CSCF node 114 b further determines whether the SIP INVITErequest message associated with the SIP INVITE response message (e.g.the modified/unmodified SIP INVITE request message originating from thefirst UE 101) had a relay address candidate for the first UE 101. If itdoes determine that the SIP INVITE request message from UE 101associated with the SIP INVITE response message had a relay addresscandidate for the first UE 101, then, in step 206, the terminatingP-CSCF node 114 b forwards the received SIP INVITE response message toanother IMS node in the IMS network 105 for routing to the first UE 101.In this example, the terminating P-CSCF node 114 b received the modifiedSIP INVITE request message from first UE 101, which had a relaycandidate address (e.g. transport address T2), so in step 206, theterminating P-CSCF node 114 b forwards the received SIP INVITE responsemessage to another IMS node for routing to the first UE 101. This meansthat a terminating IMS AGW node (not shown) is not placed in the path.

However, if the terminating P-CSCF node 114 b does determine that theSIP INVITE request message originating from the first UE 101 did nothave a relay address candidate for the first UE 101, then, theterminating P-CSCF node 114 b adds or places a terminating IMS AGW node115 b (not shown) into the path by including a second address providedby the terminating IMS AGW node 115 b as a relay candidate address intothe candidate address information for the second UE 103, which modifiesthe SDP answer in the received SIP INVITE response message from thesecond UE 103. The terminating P-CSCF node 114 b forwards the modifiedSIP INVITE response message to another IMS node in the IMS network 105for routing towards the first UE 101.

In addition, if a relay candidate address for the second UE 103 ispresent in the received SIP INVITE response message, then, in step 206,the terminating P-CSCF node 114 b forwards the received SIP INVITEresponse message from the second UE 103 to another IMS node in the IMSnetwork 105 for routing towards the first UE 101.

On receiving the SIP INVITE response message with SDP answer originatingfrom the second UE 103, the originating P-CSCF node 114 a determineswhether the second UE 101 is behind a NAT device and whether a relayaddress candidate for the second UE 103 is present in the candidateaddress information for the second UE 103 in the SIP INVITE responsemessage. If it is determined that the candidate address informationincludes a server reflexive candidate address for the second UE 103,then the second UE 103 is behind a NAT device. The originating P-CSCFnode 114 a then determines if a reflexive candidate address is present.If it is determined that a reflexive candidate address is not present inthe candidate attributes of the candidate address information, then, instep 207 a, the originating P-CSCF node 114 a modifies the candidateinformation for the second UE 103 by adding a second address (e.g. T1)provided by the IMS-AGW node 115 a as a relay candidate address for thesecond UE 103. In this example, the SDP answer of the SIP Inviteresponse message is modified such that the attribute c-line is replacedby c=T1 and the relay candidate address is included by a-line,a=candidate T1 relay. In step 207 b, the originating P-CSCF node 114 aforwards the modified SIP INVITE response message to the first UE 101.The first UE 101 uses the candidate address information for the secondUE 103 when performing ICE procedures.

If it is determined that there a relay candidate address for the secondUE 103 is present in the candidate address information for the second UE103, then the received SIP INVITE response message is forwarded to thefirst UE 101. That is, no IMS AGW node 115 a type relay addresscandidate is added. If the SDP answer did contain a relay addresscandidate then the IMS AGW node 115 a will just be visible as a relayaddress candidate in the SDP offer, not in the SDP answer. The ICEprocedures performed by the first UE 101 will then during the ICEconnectivity check phase discover the IMS-AGW node 115 a as a peercandidate.

Once the first and second UE 101 and 103 has the candidate addressinformation associated with the second and first UE 103 and 101,respectively, then these UEs 101 and 103 can, in full accordance withICE procedures, exchange ICE connectivity checks with each other todetermine the most optimal candidate pair that works for the multimediasession. If it is determined by the ICE connectivity checks that a relayserver is not needed for NAT traversal, then as illustrated in FIG. 2 b,the most optimal candidate pair is chosen allowing end to endcommunication between first and second UEs 101 and 103. If it isdetermined by the ICE connectivity checks that a relay server is neededfor NAT traversal, then, if IMS AGW node 115 a was included in the pathas outlined in FIG. 2 b, then only the IMS AGW node 115 a will beincluded into media path. The media delay has been minimized by theefficient use of communications resources such as media relay resources.

In any event, the first and second UEs 101 and 103, also calledend-points, will eventually select an optimal working candidate pair (aset of addresses). If the eventually selected candidate pair was not thedefault candidate, i.e. the address provided by IMS-AGW 115 a as thedefault relay candidate address, then the ICE controlling end point willsend an SDP update message to the originating P-CSCF 114 a, the SDPupdate message indicating which candidate pair was selected. Theoriginating P-CSCF node 114 a may then de-allocate the IMS-AGW node 115a resource, as this will not be used.

In cases where there is no IMS-AGW node 115 a required in the path forany other reason than for NAT traversal and the NAT traversal can bedone without a relay server, then if both endpoints in a session supportthe ICE based mechanism for NAT traversal based on the above solution,the session media will then go along the shortest path instead of alwaysbeing routed via a relay server. This minimises media delay for thosecases, and efficiently uses communications resources by minimisingnetwork bandwidth and media relay (IMS-AGW nodes 115 a/115 b) resources.

FIG. 3 a illustrates a flow diagram for another example solution foroperating an originating IMS P-CSCF node for ICE based NAT traversalaccording to the invention. A communications network including an IMS, afirst UE, and a second UE, is assumed in which the first UE originates acall towards the second UE. The process steps performed by theoriginating IMS P-CSCF node include:

-   A1. Receiving a SIP Invite message originating from the first UE.    The SIP Invite message includes candidate address information for    the first UE. The candidate address information may include at least    one transport address from the group of a host candidate address, a    server reflexive address, and a relay candidate address. Proceed to    A2.-   A2. Determining whether a server reflexive candidate address for the    first UE is present in the candidate address information for the    first UE. If a server reflexive candidate address for the first UE    is present in the candidate address information for the first UE,    then the process proceeds to step A3. Otherwise, if a server    reflexive candidate address for the first UE is not present in the    candidate address information for the first UE, then the process    proceeds to step A6.-   A3. Determining whether a relay candidate address for the first UE    is not present in the candidate address information for the first    UE. If a relay candidate address for the first UE is not present in    the candidate address information for the first UE, then the process    proceeds to step A4. Otherwise, if a relay candidate address for the    first UE is present in the candidate address information for the    first UE, then the process proceeds to step A6.-   A4. Modifying the SIP Invite message by including in the candidate    address information for the first UE an first address provided by    the originating IMS Access Gateway node as the relay candidate    address for the first UE. Proceed to step A5.-   A5. Forwarding the modified SIP Invite message to a further IMS node    for routing the SIP Invite message towards the second UE.-   A6. Forwarding the received SIP Invite message to the further IMS    node for routing the SIP Invite message towards the second UE

The candidate address information for the first UE is for use by thesecond UE when performing ICE procedures.

The above process may then wait for receipt of a further SIP Invitemessage from the first UE or any other UE, where it then performs theabove steps in relation to the further SIP Invite message.Alternatively, the process may proceed to that outlined in FIG. 3 b, asthe originating P-CSCF node may then expect a SIP Invite responsemessage originating from the second UE, the SIP Invite response messagebeing associated with the SIP Invite message originating from the firstUE.

FIG. 3 b illustrates a flow diagram for another example solution foroperating an originating IMS P-CSCF node for ICE based NAT traversalaccording to the invention. A communications network including an IMS, afirst UE, and a second UE, is assumed in which the first UE hasoriginated a call towards the second UE. The process steps performed bythe originating IMS P-CSCF node include:

-   B1. Receiving a SIP Invite response message originating from the    second UE, in response to a SIP Invite message from the first UE.    The SIP Invite response message includes candidate address    information for the second UE. Proceed to B2.-   B2. Determining whether a server reflexive candidate address for the    second UE is present in the candidate address information for the    second UE. If a server reflexive candidate address for the second UE    is present in the candidate address information for the second UE,    then proceed to B3. Otherwise, if a server reflexive candidate    address for the second UE is not present in the candidate address    information for the second UE, then proceed to B6.-   B3. Determining whether a relay candidate address for the second UE    is not present in the candidate address information for the second    UE. If a relay candidate address for the second UE is present in the    candidate address information for the second UE, then proceed to B6.    Otherwise, if a relay candidate address for the second UE is not    present in the candidate address information for the second UE, then    proceed to B4.-   B4. Modifying the SIP Invite response message by including into the    candidate address information for the second UE a second address    provided by the originating IMS AGW node as the relay candidate    address for the second UE. Proceed to B5.-   B5. Forwarding the modified SIP Invite response message to the first    UE.-   B6. Forwarding the received SIP Invite response message to the first    UE.

The candidate address information for the second UE is for use by thefirst UE when performing ICE procedures.

FIG. 3 c illustrates a flow diagram for another example solution foroperating a terminating IMS P-CSCF node for ICE based NAT traversalaccording to the invention. A communications network including an IMS, afirst UE, and a second UE, is assumed in which the first UE originates acall towards the second UE. The process steps performed by theterminating IMS P-CSCF node include:

-   C1. Receiving a SIP Invite message originating from the first UE in    relation to the first UE originating a call towards a second UE. The    SIP Invite message includes candidate address information for the    first UE. Proceed to step C2.-   C2. Forwarding the received SIP Invite message towards the second    UE.

The candidate address information for the first UE is for use by thesecond UE when performing ICE procedures.

The above process may then wait for receipt of further SIP Invitemessages originating from the first UE or any other UE, where it thenperforms the above steps in relation to the further SIP Invitemessage(s). Alternatively, the process may proceed to that outlined inFIG. 3 d, as the terminating P-CSCF node may then expect a SIP Inviteresponse message originating from the second UE, the SIP Invite responsemessage being associated with the SIP Invite message originating fromthe first UE.

FIG. 3 d illustrates a flow diagram for another example solution foroperating a terminating IMS P-CSCF node for ICE based NAT traversalaccording to the invention. A communications network including an IMS, afirst UE, and a second UE, is assumed in which the first UE hasoriginated a call towards the second UE. The process steps performed bythe terminating IMS P-CSCF node include:

-   D1. Receiving a SIP Invite response message originating from the    second UE in response to a received SIP Invite message originating    from the first UE. The SIP Invite response message includes    candidate address information for the second UE. Proceed to D2.-   D2. Determining whether a server reflexive candidate address is not    present in the candidate address information for the second UE. If a    server reflexive candidate address is not present in the candidate    address information for the second UE, then proceed to D7.    Otherwise, if a server reflexive candidate address is present in the    candidate address information for the second UE, then proceed to D3.-   D3. Determining whether a relay candidate address for the second UE    is present in the candidate address information for the second UE.    If a relay candidate address for the second UE is present in the    candidate address information for the second UE, then proceed to D7.    Otherwise, if a relay candidate address for the second UE is not    present in the candidate address information for the second UE, then    proceed to D4.-   D4. Determining whether a relay candidate address for the first UE    is present in the candidate address information for the first UE    from the received SIP Invite message. If a relay candidate address    for the first UE is present in the candidate address information for    the first UE from the received SIP Invite message, then proceed to    D7. Otherwise, if a relay candidate address for the first UE is not    present in the candidate address information for the first UE from    the received SIP Invite message then proceed to D5.-   D5. Modifying the SIP Invite response message by including in the    candidate address information for the second UE a second address of    the terminating IMS Access Gateway node as the relay candidate    address for the second UE. Proceed to D6.-   D6. Forwarding the modified SIP Invite response message to another    IMS node for routing the modified SIP Invite response message    towards the first UE.-   D7. Forwarding the received SIP Invite response message to another    IMS node for routing the received SIP Invite response message    towards the first UE.

The candidate address information for the second UE is for use by thefirst UE when performing ICE procedures.

FIG. 4 illustrates a schematic diagram for an example of a network node401 with P-CSCF functionality (e.g. IMS P-CSCF node 114 a and/or 114 b)for use in implementing the methods, processes and/or the solutionsaccording to the invention associated with an originating side of a calloriginated by a first UE towards a second UE. The network node 401 canbe implemented as a combination of computer hardware and software, andcan be configured to operate as an originating P-CSCF node in accordancewith the solutions described above. When operating as an originatingP-CSCF node, the network node 401 includes a receiver 402, a transmitter403, a memory 404 and a processor 405, which are connected together. Thememory 404 stores the various programs/executable files that areimplemented by the processor 405 and also provides a storage unit forany required data e.g. data representative of transport addressesassociated with one or more UEs. The programs/executable files stored inthe memory 404, and implemented by processor 405, include one or moreof, but are not limited to, an originating modify SIP Invite unit 406(e.g. orig. modify SIP Invite unit) and an originating modify SIP Inviteresponse unit 407 (e.g. orig. modify SIP Invite response unit).

The originating modify SIP Invite unit 406 includes program instructionsfor determining, on the network node 401 receiving a SIP Invite messageoriginating from a first UE (the SIP Invite message including an SDPoffer with candidate address information for the first UE), whether aserver reflexive candidate address for a first UE is present in thecandidate address information for the first UE and whether a relaycandidate address for the first UE is not present in the candidateaddress information for the first UE. When both these conditions arefulfilled, then the candidate address information for the first UE ofthe SIP Invite message is modified to include a first address providedby an originating IMS Access Gateway node as the relay candidate addressfor the first UE.

The originating SIP Invite response unit 407 includes programinstructions for determining, on receiving a SIP Invite response messageoriginating from the second UE (the SIP Invite response messageincluding candidate address information for the second UE), whether aserver reflexive candidate address for the second UE is present in thecandidate address information for the second UE and whether a relaycandidate address is not present in the candidate address informationfor the second UE. When both these conditions are fulfilled, then thecandidate address information for the second UE of the SIP Inviteresponse message is modified to include a second address provided by theoriginating IMS Access Gateway node as the relay candidate address forthe second UE.

In operation, the processor 405 and receiver 402 are configured toreceive SIP Invite messages originating from the first UE in relation tothe first UE originating a call towards the second UE, and for receivingSIP Invite response messages originating from the second UE. Theprocessor 405 and transmitter 403 are configured to transmit SIP Invitemessages originating from the first UE towards the second UE, and fortransmitting SIP Invite response messages originating from the second UEtowards the first UE.

In particular the receiver 402 is configured to receive a SIP Invitemessage originating from the first UE, the SIP Invite message includingcandidate address information for the first UE. The processor 405 isconfigured to modify the received SIP Invite message if a serverreflexive candidate address for the first UE is present in the candidateaddress information for the first UE and if a relay candidate addressfor the first UE is not present in the candidate address information forthe first UE.

If the processor 405 proceeds to modify the received SIP Invite message,then the processor 405 modifies the SIP Invite message candidate addressinformation for the first UE to include a first address provided by anoriginating IMS Access Gateway node as the relay candidate address forthe first UE. The processor 405 and transmitter 403 are then configuredto forward the modified SIP Invite message to a further IMS node forrouting the SIP Invite message towards the second UE. Otherwise, whenthe processor 405 does not proceed to modify the SIP Invite message, theprocessor 405 and transmitter 403 are configured to forward the receivedSIP Invite message to the further IMS node for routing the SIP Invitemessage towards the second UE. The candidate address information for thefirst UE is for use by the second UE when performing ICE procedures.

The receiver 402 is further configured to receive a SIP Invite responsemessage originating from the second UE. The SIP Invite response messageassociated with the SIP Invite message originating from the first UE.The SIP Invite response message includes candidate address informationfor the second UE. The processor 405 is configured to modify thereceived SIP Invite response message if a server reflexive candidateaddress for the second UE is present in the candidate addressinformation for the second UE and a relay candidate address for thesecond UE is not present in the candidate address information for thesecond UE.

If the processor 405 proceeds to modify the received SIP Invite responsemessage, then the processor 405 modifies the SIP Invite response messagecandidate address information for the second UE to include a secondaddress provided by the originating IMS AGW node as the relay candidateaddress for the second UE. The processor 405 and transmitter 403 arefurther configured to forward the modified SIP Invite response messageto the first UE. Otherwise, when the processor 405 does not proceed tomodify the received SIP Invite response message, the processor 405 andtransmitter are configured to forward the received SIP Invite responsemessage to the first UE. The candidate address information for thesecond UE is for use by the first UE when performing ICE procedures.

FIG. 5 illustrates a schematic diagram for an example of a network node501 with P-CSCF functionality (e.g. IMS P-CSCF node 114 a and/or 114 b)for use in implementing the methods, processes and/or the solutionsaccording to the invention associated with a terminating side of a calloriginated by a first UE towards a second UE. The network node 501 canbe implemented as a combination of computer hardware and software, andcan be configured to operate as a terminating P-CSCF node in accordancewith the solutions described above. When operating as a terminatingP-CSCF node, the network node 501 includes a receiver 502, a transmitter503, a memory 504 and a processor 505, which are connected together. Thememory 504 stores the various programs/executable files that areimplemented by the processor 505 and also provides a storage unit forany required data e.g. data representative of transport addressesassociated with one or more UEs. The programs/executable files stored inthe memory 504, and implemented by processor 505, include one or moreof, but are not limited to, a terminating modify SIP Invite responseunit 507.

The terminating SIP Invite response unit 507 includes programinstructions for determining, on receiving a SIP Invite response messageoriginating from the second UE in response to the SIP Invite messagefrom the first UE (the SIP Invite response message including candidateaddress information for the second UE), whether a server reflexivecandidate address for the second UE is present in the candidate addressinformation for the second UE, and whether a relay candidate address isnot present in the candidate address information for the second UE, andwhether the relay candidate address for the first UE was not present inthe associated received SIP Invite message candidate address informationfor the first UE. When these conditions are fulfilled, then thecandidate address information for the second UE of the SIP Inviteresponse message is modified to include a second address provided by theoriginating IMS Access Gateway node as the relay candidate address forthe second UE.

In operation, the processor 505 and receiver 502 are configured toreceive a SIP Invite message originating from the first UE and toreceive a SIP Invite response message originating from the second UE inresponse to the SIP Invite message originating from the first UE. Theprocessor 505 and transmitter 503 are configured to transmit SIP Invitemessages originating from the first UE towards the second UE, and fortransmitting SIP Invite response messages originating from the second UEtowards the first UE.

In particular the receiver 502 is configured to receive a SIP Inviteresponse message originating from the second UE in response to the SIPInvite message originating from the first UE. The SIP Invite responsemessage includes candidate address information for the second UE. Thetransmitter 503 is configured to forward the received SIP Invite messagetowards the second UE. The candidate address information for the firstUE is for use by the second UE when performing ICE procedures.

The receiver 502 is further configured to receive a SIP Invite responsemessage originating from the second UE in response to the second UEreceiving the SIP Invite message originating from the first UE. The SIPInvite response message includes candidate address information for thesecond UE. The processor 505 is configured to modify the received SIPInvite response message based on whether a server reflexive candidateaddress for the second UE is present in the candidate addressinformation for the second UE and whether a relay candidate address forthe second UE is not present in the candidate address information forthe second UE and whether the relay candidate address for the first UEwas not present in the associated received SIP Invite message candidateaddress information for the first UE.

If the processor 505 proceeds to modify the received SIP Invite responsemessage, then the processor 505 modifies the SIP Invite response messagecandidate address information for the second UE to include a secondaddress provided by the terminating IMS Access Gateway node as the relaycandidate address for the second UE. The processor 505 and transmitter503 are further configured to forward the modified SIP Invite responsemessage to another IMS node for routing the modified SIP Invite responsemessage towards the first UE. Otherwise, when the processor 505 does notproceed to modify the received SIP Invite response message, theprocessor 505 and transmitter 503 are configured to forward the receivedSIP Invite response message to another IMS node for routing the receivedSIP Invite response message towards the first UE. The candidate addressinformation for the second UE is for use by the first UE when performingICE procedures.

It will be appreciated by the person of skill in the art that variousmodifications may be made to the above-described examples andembodiments without departing from the scope of the present invention.For example, whilst the above-described embodiments refer to specificprotocols such as SIP signaling protocol and SDP offer/answer models, itis to be appreciated that other signaling protocols or call sessionprotocols and session description protocols may be used in place of SIPsignaling and SDP when a first UE originates a call with a second UE.Whilst the above-described embodiments refer to entities, nodes orfunctions within an IMS network, such as the IMS-AGW node(s), IMSP-CSCF(s), NAT device(s), it is possible that the names used to refer toone of more of these entities, nodes or functions, could change, or thatthe functionality of one or more of these entities, nodes, or functionsmay be combined with that of another network entity or IMS node.

Although the invention has been described in terms of example solutionsor preferred embodiments as set forth above, it should be understoodthat these examples or embodiments are illustrative only and that theclaims are not limited to only those examples or embodiments. Thoseskilled in the art will be able to make modifications and alternativesin view of the disclosure which are contemplated as falling within thescope of the appended claims. Each of the features, steps, or nodesdisclosed or illustrated in the present specification may beincorporated into the invention, whether alone or in any appropriatecombination with any other feature, step, or node disclosed orillustrated herein.

The invention claimed is:
 1. A method for operating a call originatingP-CSCF node for an Interactive Connectivity Establishment (ICE) basednetwork address translation (NAT) traversal in a communications networkincluding an IP Multimedia Subsystem (IMS), a first user equipment (UE),and a second UE, the method comprising: receiving a SIP Invite messageoriginating from the first UE, the SIP Invite message comprisingcandidate address information for the first UE; and performingoperations as follows when a server reflexive candidate address for thefirst UE is present in the candidate address information for the firstUE and a relay candidate address for the first UE is not present in thecandidate address information for the first UE: modifying the SIP Invitemessage candidate address information for the first UE to include afirst address provided by an originating IMS Access Gateway (AGW) nodeas the relay candidate address for the first UE; and forwarding themodified SIP Invite message to a further IMS node for routing the SIPInvite message to the second UE; forwarding the received SIP Invitemessage to the further IMS node for routing the SIP Invite message tothe second UE when the server reflexive candidate address for the firstUE is not present in the candidate address information for the first UEor the relay candidate address for the first UE is present in thecandidate address information for the first UE; and wherein thecandidate address information for the first UE is for use by the secondUE when performing ICE operations; receiving a SIP Invite responsemessage originating from the second UE, the second UE is behind aterminating IMS AGW node, the SIP Invite response message includingcandidate address information for the second UE; and performingoperations as follows when a server reflexive candidate address for thesecond UE is present in the candidate address information for the secondUE and a relay candidate address is not present in the candidate addressinformation for the second UE: modifying the SIP Invite response messagecandidate address information for the second UE to include a secondaddress provided by the originating IMS AGW node as the relay candidateaddress for the second UE; and forwarding the modified SIP Inviteresponse message to the first UE; wherein the first UE is behind a firstNAT device, the first NAT device being connected between the originatingIMS AGW node and the first UE, the method further comprising instructingthe originating IMS AGW node to perform address latching towards thefirst UE for discovering the transport address the first NAT device usesfor the first UE; and wherein the second UE is behind a second NATdevice, the second NAT device being connected between the terminatingIMS AGW node and the second UE, the method further comprisinginstructing the originating IMS AGW node to perform address latchingtowards the second UE for discovering the transport address the secondNAT device uses for the second UE.
 2. A method according to claim 1,further comprising: forwarding the received SIP Invite response messageto the first UE when a server reflexive candidate address for the secondUE is not present in the candidate address information for the second UEor the relay candidate address is present in the candidate addressinformation for the second UE; wherein the candidate address informationfor the second UE is for use by the first UE when performing ICEoperations.
 3. A method according to claim 1, further comprisingdetermining the address of the originating IMS AGW node for use as therelay candidate address.
 4. A method according to claim 1, whereinmodifying the received SIP Invite message further comprises setting therelay candidate address comprising the address of the originating IMSAGW node to be a default candidate address.
 5. A method according toclaim 4, further comprising: receiving an update message from the firstor second UEs being the ICE controlling endpoint, the update messageindicating that another address candidate other than the relay candidateaddress comprising the address of the originating IMS AGW node has beenselected for use; setting the another address candidate as the defaultcandidate address; and removing the address of the originating IMS-AGWnode from a media path between the first and second UEs.
 6. A method foroperating a terminating call P-CSCF node for Interactive ConnectivityEstablishment (ICE) based NAT traversal in a communications networkincluding an IP Multimedia Subsystem (IMS), a first user equipment (UE),and a second UE, the method comprising: receiving a SIP Invite messageoriginating from the first UE for the second UE, the SIP Invite messageincluding candidate address information for the first UE; and forwardingthe received SIP Invite message to the second UE; wherein the candidateaddress information for the first UE is for use by the second UE whenperforming ICE operations; and wherein the first UE is behind a firstNAT device and an originating IMS Access Gateway node and the second UEis behind a second NAT device and a terminating IMS AGW node, the methodfurther comprising instructing the terminating IMS AGW node to performaddress latching towards each UE for discovering the transport addressthe first NAT device uses for the first UE; wherein the first NAT deviceis connected between first UE and the originating IMS Access gatewaynode and the second NAT device is connected between the second UE andthe terminating IMS AGW node; and receiving a SIP Invite responsemessage originating from the second UE in response to the SIP Invitemessage originating from the first UE, the SIP invite response messageincluding candidate address information for the second UE; performingoperations as follows when a server reflexive candidate address ispresent in the candidate address information for the second UE and arelay candidate address for the second UE is not present in thecandidate address information for the second UE, and the relay candidateaddress for the first UE was not present in the associated received SIPInvite message candidate address information for the first UE: modifyingthe SIP Invite response message candidate address information for thesecond UE to include a second address provided by the terminating IMSAGW node as the relay candidate address for the second UE; andforwarding the modified SIP Invite response message to another IMS nodefor routing the modified SIP Invite response message to the first UE. 7.A method according to claim 6, further comprising: forwarding thereceived SIP Invite response message to another IMS node for routing thereceived SIP Invite response message to the first UE when the serverreflexive candidate address is not present in the candidate addressinformation for the second UE, or the relay candidate address for thesecond UE is present in the candidate address information for the secondUE, or the relay candidate address for the first UE is present in theassociated received SIP Invite message candidate address information forthe first LIE; and wherein the candidate address information for thesecond UE is for use by the first UE when performing ICE operations. 8.A method according to claim 6, further comprising determining theaddress of the terminating IMS AGW node for use as the relay candidateaddress for the second UE.
 9. A method according to claim 6, wherein thesecond UE is behind a second NAT device, the method further comprisinginstructing the terminating IMS AGW node to perform address latchingtowards each UE for discovering the transport address the second NATdevice uses for the second UE.
 10. A method according to claim 7,wherein modifying the received SIP Invite response message furthercomprises setting the relay candidate address comprising the address ofthe terminating IMS AGW node to be a default candidate address.
 11. Amethod according to claim 10, further comprising: receiving an updatemessage from the first or second UE, being the ICE controlling endpoint,the update message indicating that another address candidate other thanthe relay candidate address comprising the second address of theterminating IMS AGW node has been selected for use; setting the anotheraddress candidate as the default candidate address; and removing thesecond address of the terminating IMS AGW node from a media path betweenthe first and second UEs.
 12. A network node in a communications networkincluding an IP Multimedia Subsystem (IMS), a first user equipment (UE),and a second UE, the network node comprising: a receiver, a transmitter,a memory unit, and a processor, the processor being connected to thereceiver, to the transmitter, and to the memory unit, wherein: thereceiver is configured to receive a SIP Invite message originating fromthe first UE, the SIP Invite message comprising candidate addressinformation for the first UE; when a server reflexive candidate addressfor the first UE is present in the candidate address information for thefirst UE and a relay candidate address for the first UE is not presentin the candidate address information for the first UE, then: theprocessor is configured to modify the SIP Invite message candidateaddress information for the first UE to include a first address providedby an originating IMS Access Gateway node as the relay candidate addressfor the first UE; and the transmitter is configured to forward themodified SIP Invite message to a further IMS node for routing the SIPInvite message to the second UE; the transmitter is further configuredto forward the received SIP Invite message to the further IMS node forrouting the SIP Invite message to the second UE when the serverreflexive candidate address for the first UE is not present in thecandidate address information for the first UE or a relay candidateaddress for the first UE is present in the candidate address informationfor the first UE; and wherein the candidate address information for thefirst UE is for use by the second UE when performing InteractiveConnectivity Establishment (ICE) operations; the receiver is furtherconfigured to receive a SIP Invite response message originating from thesecond UE, the second UE is behind a terminating IMS AGW node, the SIPInvite response message including candidate address information for thesecond UE; and when a server reflexive candidate address for the secondUE is present in the candidate address information for the second UE anda relay candidate address is not present in the candidate addressinformation for the second UE, then: the processor is configured tomodify the SIP Invite response message candidate address information forthe second UE to include a second address provided by the originatingIMS AGW node as the relay candidate address for the second UE; andforwarding the modified SIP Invite response message to the first UE; andthe transmitter is configured to forward the modified SIP Inviteresponse message to the first UE; wherein the first UE is behind a firstNAT device, the first NAT device being connected between the originatingIMS AGW node and the first UE, the method further comprising instructingthe originating IMS AGW node to perform address latching towards thefirst UE for discovering the transport address the first NAT device usesfor the first UE; and wherein the second UE is behind a second NATdevice, the second NAT device being connected between the terminatingIMS AGW node and the second UE, the method further comprisinginstructing the originating IMS AGW node to perform address latchingtowards the second UE for discovering the transport address the secondNAT device uses for the second UE.
 13. A network node according to claim12, wherein: the transmitter is further configured to forward thereceived SIP Invite response message to the first UE when the serverreflexive candidate address for the second UE is not present in thecandidate address information for the second UE, or a relay candidateaddress for the second UE is not present in the candidate addressinformation for the second UE; wherein the candidate address informationfor the second UE is for use by the first UE when performing ICEoperations.
 14. A network node in a communications network including anIP Multimedia Subsystem, IMS, a first user equipment, UE, a second UE,the network node comprising: a receiver, a transmitter, a memory unit,and a processor, the processor being connected to the receiver, to thetransmitter, and to the memory unit wherein: the receiver is configuredto receive a SIP Invite message originating from the first UE for thesecond UE, the SIP Invite message including candidate addressinformation for the first UE; and the transmitter is configured toforward the received SIP Invite message to the second UE; wherein thecandidate address information for the first UE is for use by the secondUE when performing Interactive Connectivity Establishment (ICE)operations; and wherein the first UE is behind a first NAT device and anoriginating IMS Access Gateway node and the second UE is behind a secondNAT device and a terminating IMS AGW node, the processor beingconfigured to instruct the terminating IMS AGW node to perform addresslatching towards each UE for discovering the transport address the firstNAT device uses for the first UE; wherein the first NAT device isconnected between first UE and the originating IMS Access gateway nodeand the second NAT device is connected between the second UE and theterminating IMS AGW node; and the receiver is further configured toreceive a SIP Invite response message originating from the second UE inresponse to the SIP Invite message originating from the first UE, theSIP invite response message including candidate address information forthe second UE; when a server reflexive candidate address is present inthe candidate address information for the second UE, and a relaycandidate address for the second UE is not present in the candidateaddress information for the second UE, and the relay candidate addressfor the first UE was not present in the associated received SIP Invitemessage candidate address information for the first UE, then: theprocessor is further configured to modify the SIP Invite responsemessage candidate address information for the second UE to include asecond address provided by the terminating IMS AGW node as the relaycandidate address for the second UE; and the transmitter is furtherconfigured to forward the modified SIP Invite response message toanother IMS node for routing the modified SIP Invite response message tothe first UE.
 15. A network node according to claim 14, wherein: thetransmitter is further configured to forward the received SIP Inviteresponse message to another IMS node for routing the received SIP Inviteresponse message to the first UE when the server reflexive candidateaddress is not present in the candidate address information for thesecond UE, or a relay candidate address for the second UE is present inthe candidate address information for the second UE, or the relaycandidate address for the first UE is present in the associated receivedSIP Invite message candidate address information for the first UE;wherein the candidate address information for the second UE is for useby the first UE when performing ICE operations.